Loop detectors are used to detect the presence of large metal objects within a specific area. One example of a loop detector system consists of inductive loops connected to appropriate circuitry that generate a signal in the inductive loop at a set frequency which in turn creates a magnetic field in the vicinity of the inductive loop. When a large metal object comes within the range of the magnetic field generated by the inductive loop this causes a change in the inductance of the loop that in turn causes a change in the frequency in the inductive loop. This change in the frequency signals the presence of the metal object.
Inductive loops are used to detect the presence and movement of vehicles along a roadway by imbedding the loops in the pavement or asphalt of the roadway. Inductive loop systems are also used to detect the presence of vehicles within the operational area of a security gate. However problems develop when two or more independent inductive loops are placed near by each other and the operational frequency of the adjacent inductive Loops is the same in each loop, magnetic fields generated by each the loops can interfere with each other. This interference can cause false detections, i.e. an indication a vehicle is present when one is not, or false drops, i. e. an indication a vehicle has left the detection area when it has not, etc.
FIG. 1 provides a view of a sliding security gate 23 with two safety loops 25 and 27 used to detect the presence of a vehicle adjacent to either side of the gate or passing through the gate. FIG. 2 provide a view of a swinging security gate 33 and the safety loops 35 and 37 as well as the center loop 39 used to detect the presence of a vehicle adjacent to either side of the gate, within the movement area of the gate or passing through the gate. In both FIGS. 1 and 2 the loops are so close together that each will have a tendency to interfere with the operation of the other if they are operating at the same frequency.
To avoid the problem of interference between loop detectors one of the standard solutions has been to provide loop detectors with multiple selectable frequencies on which the installer has to make changes manually. This solution has its deficiencies since the technician installing the loop detector will have to check the frequency of the loop detectors with a fairly expensive instrument. Checking the frequency in this manner is time consuming. Also, the person installing the system may forget to select different operational frequencies for adjacent loop detectors or make incorrect assumptions. Another alternative is providing loop detectors that display the frequency at which the loop is operating and allow for the programming of a frequency by the technician on installation of the system. Other solutions that have been used is to not have each loop detector circuit operate continuously but rather scan at a preset time interval timed such that each loop detector circuit scans different times so that none are scanning at the same time.
Thus, what is needed is a system and method for preventing cross talk between the inductive loops of adjacent loop detection circuits. A system and method that can automatically detect if two adjacent loops have the same operational frequency and set the oscillatory frequency of each loop detection unit to a different frequency that will result in a non conflicting working frequency between them. Such a system must be simple cost effective and easy to install in new systems as well as retrofit on to existing systems.